Authentication of items such as documents, especially banknotes (currency) and the like, against forgery or counterfeiting may involve detection of the exponential decay from photoluminescence. In particular, the intensity y of time resolved emissions from quasi-resonant materials, which emit light having a similar wavelength to the excitation source, tend to be dominated by an exponential function of time (t) having the general formy=Ae−t/τ  (1)where A (the amplitude) describes the intensity of the signal at time t and τ (the lifetime of the decay) provides an identifier for the specific quasi-resonant luminescent material. Security features including such materials, though proven, are growing more ubiquitous and therefore less reliable for authentication of very high security features such as those on banknotes and security documents.